Developments in internal combustion engine technology have shown that compression ignition engines, commonly referred to as diesel-cycle engines, can be fuelled by gaseous fuels instead of liquid diesel fuel without sacrifices in performance or efficiency. Examples of such gaseous fuels include natural gas, methane, propane, ethane, gaseous combustible hydrocarbon derivatives and hydrogen. Using such gaseous fuels instead of liquid diesel fuel generally results in cost, availability and emissions benefits.
By injecting a gaseous fuel directly into an engine's combustion chamber near the end of the compression stroke, it is possible to achieve substantially the same performance and efficiency as a diesel engine. However, a challenge of this approach is that the gaseous fuel must be delivered to the combustion chamber at a pressure that overcomes the high in-cylinder pressures that are present during this part of the engine cycle. To practice this method, gaseous fuel pressure is preferably between at least 17 and up to 70 MPa (between at least 2500 psi and up to 10,000 psi).
Conventional gaseous-fuelled engines which are based on the Otto cycle typically inject the gaseous fuel into the intake manifold where the gaseous fuel can be premixed with the intake charge. The pressure within the intake manifold is much lower than the pressures that can occur within the combustion chamber, so pressure within the gaseous fuel supply system are normally less than 0.7 MPa (about 100 psi). By associating the gaseous fuel injection valve with the intake manifold, the fuel supply system components and the seals between components are not exposed to the higher temperatures that can occur nearer to the combustion chamber. However, present day gaseous-fuelled engines have been unable to match the performance and efficiency of diesel-fuelled engines.
With present day gaseous fuelled engines there is no need to supply gaseous fuel at pressures above 17 MPa (about 2500 psi) and no one has addressed the problem of sealing connections between components to prevent the leakage of a gaseous fuel supplied at such pressures, especially for an application where the gas pressure can also quickly fluctuate between very high pressures and much lower pressures.
For example, during normal operation of an internal combustion engine, if the gaseous fuel is injected directly into the combustion chamber, fuel pressure in a fuel supply system can vary within a range of pressures between 17 MPa and 70 MPa and changes in pressure can occur with a frequency of between 1 and 10 hertz. When such an engine is shut down, the fuel supply system can be vented, rapidly reducing the pressure from operating pressure to atmospheric pressure. Upon shut down, while it is desirable to slow the rate at which fuel pressure is reduced, fuel pressure can still fall from maximum pressure to about atmospheric pressure in less than 20 seconds, and more commonly between 1 and 8 seconds.
Conventional diesel engines inject liquid diesel at even higher pressures than engines fuelled with gaseous fuel because fuel pressure is employed to vaporize the liquid fuel. In a modem diesel-fuelled engine, the liquid fuel can be introduced into an engine's combustion chamber at an injection pressure that is between 70 and 207 MPa (between about 10,000 psi and 30,000 psi). New diesel engines are being introduced with even higher fuel injection pressures to improve atomization for reduced emissions.
Seals used in diesel fuel systems comprise o-ring seals that are commonly made from fluoroelastomers. An example of a suitable fluoroelastomer is the product sold by DuPont Dow Elastomers LLC under the tradename Viton®. Buna-N Nitrile Rubber can also be used.
Seals comprising fluoroelastomers and Buna-N Nitrile have been tested for sealing connections between components of a high-pressure gaseous fuel system that is operable with a maximum gaseous fuel pressure between 20 MPa (about 3000 psi) and 42 MPa (about 6000 psi). These seals were found to fail after a short time. It is believed that the pressure fluctuations between operating pressure and atmospheric pressure was the primary cause of failure. The failure mode was consistent with the characteristics of explosive decompression, which can occur when a material is subjected to rapid changes in gas pressure. Because the molecular size of fuel gas constituents is much smaller than the molecular size of liquid fuel constituents, a significant amount of gaseous fuel can be absorbed into a seal member when it is exposed to high-pressure gaseous fuel. When the fuel lines are vented, or gaseous fuel pressure is otherwise rapidly reduced, the gaseous fuel is released from the o-ring seal and the o-ring seal material can break to allow the absorbed gaseous fuel to escape. This understanding of the seal failures explains why such seal failures are not encountered when the same seal material is used for conventional liquid fuel applications when the seals can be exposed to even higher pressures.
The problem of explosive decompression and the resulting failure of resilient static or dynamic seals does not occur in all pressurized fluid systems. For example, in environments where rapid reductions in gas pressure can be avoided, explosive decompression is not a problem. The problem of explosive decompression can occur when the fluid is a gas, which is at relatively high pressures and when the system is subjected to rapid pressure fluctuations. Accordingly, explosive decompression is not a common problem. There are other variables that can influence the susceptibility of a material to failure because of explosive decompression, such as the porosity of the material, and the ability of the gas to be absorbed into the pore volume at system pressure. Manufacturers of seals do not normally rate a seal material's effectiveness against explosive decompression, making difficult the selection of an appropriate seal material.